METHOD FOR FEEDING A PLURALITY OF DISCHARGE OR FLUORESCENT LAMPS CONNECTED IN SERIES, AND APPARATUS THAT CARRIES OUT SUCH METHOD

Abstract: A plurality of electric discharge lamps, or fluorescent lamps, connected in series, are fed at the startup with a variable voltage having a frequency that is varied, during a preset time period, between preset extreme values, respectively a minimum value, and a maximum value, so as to cause the startup of the lamps with a convenient voltage value. Subsequently the lamps are fed with constant current having nominal frequency.

Description: METHOD FOR FEEDING A PLURALITY OF DISCHARGE OR FLUORESCENT LAMPS CONNECTED IN SERIES, AND APPARATUS THAT CARRIES OUT SUCH METHOD TECHNICAL FIELD The present invention is concerned with the technical field relating to continuous feeding of lamps for street, square or generic areas lighting, where the current (fed to the lamps pet) is kept at a value within a preset range . These lamps are gas-discharge or luminescence lamps, wherein the light emission is produced by an electric discharge that is made to occur in a gas, in a metallic vapour, or in a gas and vapour mixture. As an example the mercury or sodium vapour lamps and the like, must be cited, and they may have a low or high pressure. BACKGROUND ART Apparatuses for carrying out the startup and subsequently feeding the lamps as previously mentioned are already known. In an apparatus there is provided a bridge circuit having branches formed by reactive components. More precisely, two opposed branches of the bridge circuit include similar capacitors, while the remainders include similar inductances. A diagonal of the bridge circuit is shunted from the secondary winding of a feed transformer while the other diagonal is formed by the load constituted by a plurality of lamps . The values of the inductances and capacitors are such that the well known resonance condition occurs. Consequently, if the capacitive and inductive reactances are considered as ideal ones (that practically is, if the resistive value is made negligible with respect to the reactive value) , the current intensity value that run through the lamps will practically be independent from the impedence value of the load. This current value will be a function of the feed frequency and voltage. A further known apparatus includes a feed transformer ( , also known as constant current transformer) that has the secondary winding mobile with respect to the primary winding. The secondary winding is subjected to the action resulting from the thrust of a counter-weight combined with the repulsion acting between the primary and secondary windings. The equilibrium condition for the mobile winding, in which a preset distance of this latter from the primary winding is set (as well as a corresponding secondary reactance) corresponds to a preset secondary current value that constitutes the feed current for the lamps. Any current variation provoked by any cause (such as a variation of the feed voltage, or a variation of the number of lamps connected) causes the mobile windings to move until the above mentioned preset value is reset for the current through the lamps. At the lamps startup, both the apparatuses just described must supply a voltage that is about three times higher than the nominal operational voltage of the lamps that are fed. It is known that at the start of the electric arch that occur in each lamp, the current raises up to a remarkable value and this current must be limited to avoid damages to the lamps. In the first apparatus the feed voltage is progressively adjusted, according to discrete steps. This is obtained (for example) by means of a multiple connection secondary winding in the feed transformer. The adjustment operation is carried out by relatively complex means . In the second apparatus described above the current value is limited due to the quick movement of the mobile winding away from the stationary winding. This quick movement increases the reactance value in the secondary winding, that is to say the secondary impedence, thus limiting the starting current. Therefore, in the known apparatuses there is the need of supplying a feed voltage of such value so as to start the arch, and this value may be maintained only during the lamps startup operation. It must be noted that in the known apparatuses the starting current limitation occurs after a certain delay. In other words the actual effect results in the fact that the time interval during which the starting current occurs is limited. A lack of the feed voltage causes another problem, besides turning the lamps off, because in order to reset the condition that allows a new startup for the discharge lamps, it is necessary that the lamps be kept off for several minutes, before feeding them again. DISCLOSURE OF INVENTION The object of the present invention is to propose a method by means of which a plurality of discharge or fluorescent lamps, connected in series are fed, and by means of which the startup voltage is considerably lowered with respect of the startup voltage that is required in the known apparatuses, this resulting in a limitation of the startup current. A further object of the present invention is to propose an electric-electronic apparatus that carries out the above mentioned method and that allows for maintaining an operational pre-determined feed current for the lamps, this feed current being independent from any variation of the number of lamps which are to be fed, independent from any variation in the supply network voltage frequency or in the frequency of the voltage delivered by a generator group, and independent from any voltage variation in either one or the other voltage supply line. A further object of the present invention is to propose an apparatus such that it can be controlled by an operating central unit that is in turn controlled by a microprocessor. The above mentioned objects are achieved in accordance with the content of the claims. BRIEF DESCRIPTION OF DRAWINGS Characteristic features of the invention are highlighted in the following, particularly referring to the accompanying drawing wherein there is shown a possible block diagramm of the electric-electronic apparatus that carries out the method proposed herein. BEST MODE FOR CARRYING OUT THE INVENTION Referring to the said drawing, reference numeral 1 indicates an AC/DC converter that includes a three-phase rectifier bridge circuit 2 that can can be fed by a power supply three-phase line after that a switch 4 has been turned on. Switch operating means are controlled by an operating central unit that is indicated by numeral 10 in the drawing and only schematically shown as it is not part of the present invention. Conductor wires 3a,b,c of the supply line 3 include fuses 6. One of this conductor wire (the one indicated by 3c, for example) along with the neutral wire 3d, have the task of feeding auxiliary devices (which are generally indicated with numeral 7) , when the related switch has been toggled to on. The supply line 3 also includes a radio frequency filter 9 situated upstream of the switch 4. The radio frequency filter 9 will be better described in the following. At the converter 1 output there is an unidirectional voltage VI. This unidirectional voltage is applied to the input of a filter 11, that includes for example an inductance 11a (with a fuse connected in series therewith) and a capacitor lib connected to the output of the filter. The unidirectional voltage V2, that results at the filter output, has a ripple factor lower than the ripple factor of voltage VI . Numeral 13 indicates an AC/DC converter that is fed via an inductance 14 either with the unidirectional voltage V2 supplied by the filter 11 or with the direct current resulting from the voltage V of a second power supply line 15. The second supply line 15 includes a switch 16 that is normally kept on, and can be operated by means of a controlled diode 17 having the gate 17a connected to the unit 10. The second converter 13 is fed by either one or the other power supply line so as to assure a continuous operation of the devices situated downstream thereof (which are listed in the following) . In normal condition power is supplied by the three-phase supply line 3, while the supply line 15 is not used. If the supply line 3 is disactivated (this event being detected by conventional sensor means, which are not shown and are connected to the unit 10) the switch 4 will turn off and the diode 17 will be simultaneously activated. In this way the supply line 15 will be activated while the three-phase supply line 3 will be kept inactive until the causes which provoked the break-down are eliminated. The converter 13 , that is for example a transistor type one, is controlled by the unit 10, in accordance with known procedures and with the help of conductors which are indicated by numeral 13a. The effective value and frequency of voltage V3 at the output of the converter 13 may be adjusted, in stepless way, as a function of commands emitted by the unit 10. The voltage V3 is made substantially sinusoidal by means of a filter 18 that is connected in cascade to the converter 13. The result is a voltage V4. This voltage V4 is supplied to the primary winding of a transformer 19 that has the task of adapting the load defined by the total impedence of a plurality of lamps 20, which are electric discharge (or luminescent) lamps connected in series, used for example for lighting of streets, squares, or other ares for generical purpose. Discargers 23 are shunted from each conductor coming from the secondary winding of the transformer 19, and the earth terminal. The voltage emitted by the secondary winding is measured by a proper voltage transformer (VT) 22. If there is the necessity, a voltage capacitive divider will be used. The current I that passes through the lamps 20, is measured by means of conventional current transformer (CT) . The result of the measurements are delivered to the unit 10. Data supplied by conventional adjustment means 30, are sent to the unit 10; the value of the current I can be set by means of the adjustment means 30. The unit 10, as well as the adjustment means 30, may be advantageously controlled, if required, by means of an operating group 50, so that it is possible to check the operation of the devices making up the apparatus, even from a remote place (and it is made possible to certify the check result) . The method, according to which the apparatus operates, is described in the following. At the startup the unit 10 changes the frequency of the voltage V3 at the output of the converter 13. The voltage V3 frequency is varied in a range comprised between minimum and maximum values (for example in a range comprised between 50 Hz and 400 Hz) , for a time interval of preset duration (for example, 200 ms; this value is indicated as an example and has not any limitative effect) . This method allows to considerably make lower the arch start voltage with respect of the start voltage requested in operation with known methods. The voltage, as estimated by the applicant in recent experiments, is about 20-30 % lower than the voltage in the previously used methods. This is a surprising result that brings about extremely important advantages in that, besides reducing the start voltage value, it also consequently requires a lower start current, with all the advantages that this implies as far as duration and operation of the lamps are concerned. It is obvious that for lamps which have nominal operation frequency of 60 Hz, the minimum voltage frequency value at the startup is equal to the nominal operation value. The frequency variation in the above mentioned time interval can be carried out in accordance with any frequency-time function rate, for instance in accordance with a linear rate. From experiments conducted by the applicant, it has been found out that, the method as indicated hereinabove can be carried out through one time interval only. If it is necessary, the startup will be obviously carried out through a sequence of time intervals which may be different from one another. In each interval, the frequency value for the voltage V3 is varied between minimum and maximum values which are different from one another or equal to each other in the variuos time intervals . After the startup, the frequency of the voltage V3 is set at a value equal to the nominal frequency value of the lamps 20. The effective value of the voltage at the output of the transformer 19 is such that the current through the lamps coincides with the nominal current thereof. The subject apparatus can quickly change the effective value of the voltage V3, in accordance with a specific request, so as to change the brillance of the lamps. The adjustment can be performed by the adjustment means 30. Harmonic caused by the absorption of the DC/AC converter 13, are effectively filtered by the inductance 14 (which is particularly proficuous when the power supply line 15 is used) . The proposed apparatus allows to feed the lamps 20 also when there is no voltage along the power supply line 3, and this eliminated any inconvenience provoked by the lamps being turned off during normal operation time. When the apparatus is turned off, the energy stored by the inductance 14 is dissipated on the resistor 25. This is made possible by the consequent direct polarization of the diode 26 that is connected in series with the resistor 25. It is known that operation of a rectifier bridge requires intermittent current absorption. In the present case, the radio frequency filter 9 effectively smoothes the harmonic oscillation in the current throught the conductor wires 3a,b,c, thus avoiding any interference in the supply lines. It must be noted that the capacitor lib of the filter 11, besides the task already cited, also allows for the recovery of reactive energy absorbed by the components located downstream therof (mainly the transformer 19, the current and secondary voltage measuring means 21 and 22) . In other words, the power factor of the circuit is corrected. In the accompanying drawing, the power supply line 3 is a three-phased one. It is obvious that this power supply line (that is connected either to the power supply network or to a power generator group) may also be mono-phased. INDUSTRIAL APPLICABILITY The apparatus described herein carries out the proposed method and moreover assure a continuous feeding to the lamps, with a constant current. This operation does not feel the effects provoked by the following causes: - variation of the total impedence of the lamps (for example when one or more lamps go out of order during operation) ; - variation of the voltage along the three-phase power supply line 3 (or, on the other hand, variation of the voltage along the power supply line 15) ,* - variation of the three-phase power supply line 3 voltage frequency (obviously, when in use) ; - switching from one to the other power supply line 3,15. To sum up, the apparatus described herein brings to the following advantages: - the lamp startup is carried out in accordance with the proposed method; - the service (feeding of the lamps) is carried out continuously; - the load is subdivided in a well balanced way on the three phases (when the supply line 3 is a three-phased one) ; - the power factor is kept near to one with any load and current value I; - the brillance of the lamps can be adjusted in a stepsless way; - the shape of the wave for the current through the lamps is sinusoidal, with a consequent reduction of the energy that is irradiated with radio frequency; - the brillance of the lamps does not change, even if load variations, or feed voltage and supply network frequency, or variation in the voltage from a generator occur; - a high efficiency is obtained; - it is possible to insert an interface between the unit 10 and/or the operating group 50 (if this is present) this interface including a PLC or a micro-processor. Finally, it must be highlighted that the space required by the proposed apparatus is not larger than the space required by the known apparatuses described in the introductory statement.

Claims: CLAIMS 1. Method for feeding a plurality of discharge or fluorescent lamps connected in series, characterized in that it includes feeding these lamps (20) , at the startup, with a frequency that is varied, at least during a preset time interval, in a range comprised between extreme preset values, respectively a maximum value and a minimum value, so as to determine, for a convenient voltage value, the startup of these lamps, and feeding these lamps (20) , after that the startup has been completed, with alternate voltage having frequency equal to the nominal frequency value of these lamps, this last voltage having a value such as to keep a preset feed current through the same lamps. 2. Method according to claim 1, characterized in that the said frequency minimum value coincides with the frequency nominal value for these lamps. 3. Electric-electronic apparatus for carrying out the startup of a plurality of discharge or fluorescent lamps connected in series and for feeding these lamps uninterruptedly, this apparatus being connected with two power supply lines (3,15) , respectively a first power supply monophase or three-phase line, and a second direct voltage supply line, the apparatus including an operating central unit, measuring means (21) connected to said unit (10) for measuring the value of the current passing through said lamps (20) , and adjustment means (30) electrically connected to said unit and aimed at regulating the value of said current; the said apparatus being characterized in that it also includes: a first converter (1) that can be connected to said first supply line (3) when a related switch (4) controlled by said unit (10) , is turned on, this converter having the task of outputting a unidirectional voltage (VI) ; a first electric filter (11) , connected in cascade to said converter (1) and aimed at reducing the ripple factor value for said unidirectional voltage; a second converter (13) , operated by said operating central unit (10) and having the input either connected to the output (V2) of said filter (11) , or connected to said second power supply line (15) , as a consequence of said switch (4) being turned off, and of switching means (17) being activated, said switching means (17) being set on the said second power supply line (15) and controlled by said unit (10) , the output of said second converter (13) being connected to an adapter transformer (19) aimed at adapting the load formed by the total impedence of the said lamps (20) , said second converter (13) being designed to output a variable voltage (V3) with a frequency that is varied at the startup of the said lamps between two extreme values at least during a preset time interval, these extreme values being determined, along with the duration of the said time interval, by the said operating unit (10) , so as to cause the startup of the said lamps (20) with a convenient voltage (V3) value, the said second converter (13) having also the task of outputting, after that the startup has been completed, the said voltage (V3) with an effective value made variable in a stepless way to define a current (I) through the said lamps (20) having a value equal to the value set by means of said adjusting means (30) and independent from any variation of the said impedence, from any variation of the said mono-phase or three-phase supply line (3) voltage frequency, and lastly from any variation of the voltage value of either one or the other line (3,15) . 4. Apparatus according to claim 3, characterized in that it includes a second filter (18) put between the output of the said second converter (13) and the primary winding of the said transformer (19) , this second filter being aimed at making almost sinusoidal the feed voltage (V4) supplied to the said primary winding. 5. Apparatus according to claim 1, characterized in that the said first filter (11) includes a capacitor connected to the output of the same first filter, this capacitor having a capacity (lib) such as to correct at least partially the power factor for the load existing downstream thereof. 6. Apparatus according to claim 4, characterized in that it includes a radio frequency filter (9) connected to said mono-phase or three-phase line (3) . 7. Apparatus according to claim 4, characterized in that it includes : an inductance (14) that is passed through by the same current directed to the said second converter (13) ; a resistor (25) and a diode (26) , connected in series to each other and connected in parallel to said inductance (14) , the cathode of the said diode being connected to the positive terminal of the said first filter (11) .

Inventors: BASILE GIANLUIGI, BASILE, GIANLUIGI
Applicants: BASILE GIANLUIGI, OCEM SPA, BASILE, GIANLUIGI, O.C.E.M. S.P.A.
Application year: 1994
Main classification: H05B41
IPC codes: H05B41